Scientific program: information on
Working Groups

The scientific work in this COST action
will be conducted by four working groups. On this page you will find
general information on the research topics and interests. More detailed
information is given on separate pages of the working groups.

Early and sensitive
detection and diagnosis of phytoplasmas is of paramount importance for
effective prevention strategies, particularly because phytoplasmas may
have a very long latency period. The main objectives of this WG are to
compare diagnostic procedures already available for most phytoplasma
pathogens and/or develop novel methods and integrate these into
sensitive and simple early detection protocols, suitable for monitoring
propagation material and for screening in plant-inspection services. To
accomplish the goals in this task marker genes that show sufficient
polymorphism will be selected as DNA bar-coding regions, and a database
of available collections of phytoplasma strains and/or DNA will be
established.

Epidemiology will
study the dispersal of phytoplasma diseases. Phytoplasmas are
transmitted in a persistent manner by insects belonging to the families
Cicadellidae, Cixidae, Psyllidae, Delphacidae,
and Derbidae. The vector acquires the phytoplasma by feeding on
an infected plant and then transmits the pathogen to a healthy one only
after completion of the latent period, during which phytoplasmas
multiply in the midgut, haemocytes and salivary glands of the vector.
Factors influencing the length of these periods as well as the
efficiency of transmission will be studied. Once a vector becomes
infectious, infectivity is retained for life, although some
discontinuities in vectoring abilities have been reported for several
phytoplasma-vector associations that will also be investigated. Some
factors influencing transmission, among which are life stage, gender,
presence of associated symbionts, flight behaviour, weed control
measures, temperature, phytoplasma strain, source and recipient plant
species will be studied for relevant phytoplasmas. Pathogenicity effects
on different organs or even reduction of longevity and fecundity will be
studied for selected phytoplasma-infected vectors. Although phytoplasmas
have been detected in various organs and tissues of the vectors, the
existence of two barriers has been suggested: the midgut and the
salivary glands. There are reports of phytoplasma multiplication in the
midgut of nonvector insects, clearly indicating that there are cases
where phytoplasmas colonize the insect but are not transmitted. Moreover
in some cases, even the host plant may influence the outcome of
transmission. In fact certain plant species may be infected with
phytoplasmas by feeding insects, but are unsuitable for further
acquisition, at least with some vector species.
Micropropagation together with other agricultural practices such as
grafting, cutting, stool bed and other systems to propagate plant
germplasm avoiding sexual reproduction are other known ways for
transmitting phytoplasma diseases, and recently the possibility of
transmission through seed has also been under investigation. In apple,
transmission by natural root bridges may be of underestimated importance
as well.
The objectives of this WG are to establish a vector monitoring system
throughout Europe to identify phytoplasma vector species, monitor their
spread throughout the COST countries, and to coordinate research into
these and other means in which phytoplasmas are spread.

Control of epidemic
outbreak can be carried out theoretically either by controlling the
vector or by eliminating the pathogen from the infected plants by
antibiotics (mainly tetracycline, due to the lack of a cell wall in
phytoplasmas) or by other chemicals. However, these protection measures
have proved to be quite ineffective under field conditions, firstly
because it is impossible to eliminate all vectors from the environment,
and secondly because the use of antibiotics is very expensive, not
allowed in several countries, and not always effective over the
long-term since they do not eradicate the phytoplasmas, such that
repeated treatments are necessary. Therefore the only effective way to
control phytoplasma infection has been to prevent the outbreaks by
ensuring that clean planting material is used, or by endeavouring to
find and/or breed varieties of crop plants that are resistant or
tolerant to the phytoplasma/insect vector. In order to advance this
field of research basic knowledge about the epidemiology, the
pathogenicity mechanisms of the phytoplasmas, the effects of
environmental factors on disease and symptom development, and the nature
of resistance/tolerance in host plants is required. The recent
sequencing of phytoplasma genomes has provided evidence that small
peptides secreted by phytoplasmas are able to enter plant cells and move
between cells, and that some of these secreted peptides are likely to be
key pathogenicity factors, and may therefore be potential targets for
plant defence mechanisms. Identification of alternative control
strategies against these diseases, such as the possibility to use
biocontrol organisms or phytoplasma mild strains could also provide
innovative and promising tools for limiting phytoplasma spread in an
environmentally sustainable approach. Studies on microorganisms as
potential biocontrol agents or plant resistance inducers have given
promising results. For example, bacterial symbionts that might be able
to reduce phytoplasma transmission by leafhoppers have been identified.
Reduced symptom expression in phytoplasma-infected plants treated with
arbuscular mycorrhizal fungi, and the capacity of two fungal elicitins
to prevent symptom expression in tobacco plants infected with stolbur
phytoplasmas, were recently reported. In addition, the occurrence of
mild strains of phytoplasmas might allow for disease control through
cross protection. This WG will coordinate the results from
epidemiological and molecular studies to formulate new and improved
strategies for the control and management of phytoplasma diseases.

Over the
past 5 years, European research teams have been involved in a number of
phytoplasma full genome sequencing projects and some of this sequence
information is available in public access databases. These projects have
resulted in major advances in understanding phytoplasma genomics. The
genomes themselves encode between 496 and 839 genes, and have very low
G+C content (23- 29.5 mol%). Whilst the main housekeeping genes appear
to be conserved among phytoplasmas, there are also other genes that are
unique to specific strains. Compared to other organisms, phytoplasmas
lack genes encoding components of the pentose phosphate pathway, lack
most genes for nucleotide synthesis, and also lack genes for the
F0F1-type ATP synthase, which was previously thought to be a component
of the minimal gene set required for all living organisms. Studies are
currently identifying the various biosynthetic pathways that exist in
phytoplasmas, and the transport mechanisms that are involved in
importing essential compounds from host plants and insects, and in
exporting potential pathogenicity factors into these hosts. In addition,
there have been a number of studies to examine the changes in host gene
expression that occur in infected plants, and the physiological and
metabolic changes that occur in these hosts. Such studies have involved
the use of differential display, cDNA-AFLP and microarrays technologies
in a range of plant hosts, such as Arabidopsis, tomato, apple, pear,
plum, Catharanthus roseus and poinsettia, and a number of upand
down-regulated plant genes have been identified in these different
systems.